1. Field of the Invention
The present invention relates to a technique for placing a digital word or private key into an integrated circuit in a manner where it cannot be later read or modified from outside the integrated circuit in order to provide a secure communication system.
2. Description of the Prior Art
The security of data communications has been increasingly important especially where sensitive information is to be transmitted to one or more remote locations over shared lines or channels. Typical of such sensitive information are files, programs and business information of a corporation or possible government communications.
A microcomputer cipher apparatus for secure transmission is disclosed in U.S. Pat. No. 4,249,180 issued to G. Eberle et al on Feb. 3, 1981. The disclosed apparatus includes a microcomputer that controls a cipher process by which sensitive data is encrypted before transmission. As shown in FIG. 6 of the patent and described in the specification, two basic levels of cipher keys are provided by and are stored in a cipher apparatus. As shown in FIG. 6 of Eberle et al, the cipher keys are stored in programmable read only memories (PROMs), and an operator can enter a new key at any time by using an "Enter Key".
Secure encryption for communication is also now available on a single VLSI device as shown on the Motorola Semiconductor Data Sheet for the MC6859 Data Security Device. The MC6859 Data Security Device is a monolithic MOS integrated circuit for use in equipment requiring protection of data. The cryptographic algorithm used by this device is the Data Encryption Standard (DES) as adopted by the U.S. Department of Commerce, National Bureau of Standards. With this device the key is placed into the device when a transmission is to be made. The use of a secondary key is also possible for increased security and the secondary key is transmitted to the remote location in encrypted form.
For these devices, the private key is externally applied. This implies that the key is stored in a ROM or set of switches and is accessible to anyone who can examine the security hardware. The problem remaining in the prior art is to provide a data encryption device or integrated circuit which once the key is entered cannot be read or modified from outside the device or integrated circuit to provide a secure communication system.